Question

In ${C_3}$ cycle for the fixation of every $C{O_2}$ molecule, the reduction and regeneration steps require
A. $3ATP$ and $\;2NADP{H_2}$
B. $2ATP$ and $\;2NADP{H_2}$
C. $2ATP$ and $\;3NADP{H_2}$
D. $3ATP$ and $\;3NADP{H_2}$

Answer 1

The Calvin cycle or ${C_3}$ cycle, is the most extensive $C{O_2}$ fixation pathway among autotrophs. In ${C_3}$ cycle, for the fixation of each carbon dioxide molecule, the reduction and regeneration stages need about $3ATP$ and $\;2NADP{H_2}$ molecules.

Complete answer:
The Calvin cycle is also known as Photosynthetic Carbon Reduction (PCR) cycle. The Calvin cycle is the cycle of chemical reactions performed by plants to fix carbon from $C{O_2}$ into three-carbon sugars. It is divided into three major steps that are as follows:
Carboxylation (Carbon fixation): zero $ATP$ and zero $NADP{H_2}$
Reduction (Glycolytic reversal): two $ATP$ and two $NADP{H_2}$
Regeneration (Regeneration of RUBP): one $ATP$ and zero $NADP{H_2}$

Thus, each turn of the ${C_3}$ cycle uses up $3ATP$ and $\;2NADP{H_2}$ in the processes of reducing (adding electrons to) 3-phosphoglyceric acid to produce glyceraldehyde-3 phosphate, and regenerating $RuBP$ so that they can accept a new atom of carbon from $C{O_2}$ from the air.

So, the correct answer is '$3ATP$ and $\;2NADP{H_2}$​'.

Additional Information:
By using the energy carriers created in the initial stage of photosynthesis, the Calvin cycle reactions fix $C{O_2}$ from the air to build carbohydrate molecules. An enzyme, $RuBisCO$, catalyzes the fixation reaction, by combining $C{O_2}$ with $RUBP$. The resultant six-carbon compound is broken down into two three-carbon compounds and the energy in $ATP$ and $NADPH$ is used to convert these molecules into$G3P$. One of the three-carbon molecules of $G3P$ leaves the cycle and becomes a part of a carbohydrate molecule. The remaining $G3P$ molecules keep on in the cycle to be formed back into $RUBP$, which is ready to react with more $C{O_2}$.

Note: Calvin cycle reactions are also considered as the light-independent reactions of photosynthesis as they are not directly powered by light. Instead, the Calvin cycle is powered by $ATP$ and $NADPH$, which are created by harnessing the energy from photons in the light-dependent reactions.